Method for forming a breathable film

ABSTRACT

A method for forming a breathable film material. A film which is a blend of a thermoplastic polymer and an inorganic filler is fed between a first roll and a second roll. The first roll has a plurality of toothed regions spaced apart by a plurality of grooved regions. The toothed regions include a plurality of teeth. The second roll has a plurality of teeth which mesh with the teeth on the first roll. As the film is fed between the first roll and the second roll the portion of the film passing between the grooved regions on the first roll and the teeth on the second roll remains substantially planar forming first regions in the film while the portion of the film passing between the teeth on the first roll and the teeth on the second roll are formed into second regions having raised rib-like elements.

FIELD OF THE INVENTION

The present invention relates to a method for forming a breathable film,and more particularly, to a method for forming a breathable filmcomprising a blend of a thermoplastic polymer with an inorganicmaterial. The breathable film is particularly useful as a backsheet on adisposable absorbent article.

BACKGROUND OF THE INVENTION

Infants and other incontinent individuals wear disposable absorbentarticles such as diapers to receive and contain urine and other bodyexudates. Absorbent articles function both to contain the dischargedmaterials and to isolate these materials from the body of the wearer andfrom the wearer's garments and bed clothing. Disposable absorbentarticles having many different basic designs are known to the art. It isalso known that the exterior of disposable diapers can be covered with aflexible, liquid and vapor impervious sheet to prevent any absorbedliquid from passing through the diaper and soiling adjacent articlessuch as clothing, bedding and the like. These outer covers, generallyreferred to as backsheets, are often constructed from fluid imperviousfilms such as polyethylene. Although such backsheets do prevent liquidfrom passing through the diaper, they also can make the diaper feel hotand uncomfortable to wear because of their impermeability to air and/ormoisture.

Backsheets which are pervious to vapor are generally known as breathablebacksheets and have been described in the art. In general, thesebacksheets are intended to allow the passage of vapor through them whileretarding the passage of liquid. For example, U.S. Pat. No. 3,156,242issued to Crowe, Jr. on Nov. 10, 1964 teaches the use of a microporousfilm as a breathable backsheet. U.S. Pat. No. 3,881,489 issued toHartwell on May 6, 1975 teaches a breathable backsheet comprising incombination two layers, the first of which is a low void volumeperforated thermoplastic film and the, second of which is a porous highvoid volume hydrophobic tissue. U.S. Pat. No. 3,989,867 issued to Sissonon Nov. 2, 1976 teaches a breathable backsheet provided with taperedhollow bosses which prevent the passage of liquids while allowing vaporsto readily pass.

Of the breathable backsheets, microporous films are most commonly used.Microporous films typically comprise a blend of a thermoplastic polymerand an inorganic filler such as calcium carbonate. The blend undergoespore formation upon stretching as the inorganic filler separates fromthe polymer due to stress concentration. The formation of microporespermits the film to be breathable allowing the passage of vapor throughthe micropores while retarding the passage of liquid.

It is an object of the present invention to provide a novel process forforming a breathable film from a blend of a thermoplastic polymer withan inorganic material.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method forforming a breathable film. The film is comprised of a blend of athermoplastic. polymer with an inorganic material being selected fromthe group consisting of calcium carbonate, clay and titanium dioxide,with the preferred inorganic filler being calcium carbonate. The blendedfilm is fed between a pair of opposing first and second rolls. The firstroll comprises a plurality of toothed regions spaced apart by aplurality of grooved regions wherein the toothed regions comprises aplurality of teeth. The second roll comprises a plurality of teeth whichmesh with the teeth on the first roll. As the film is fed between thefirst and said second rolls the portion of the film passing between thegrooved regions on the first roll and the teeth on the second rollremains substantially planar forming a first region in the film whilethe portion of the film passing between the teeth on the first roll andthe teeth on the second roll is formed into second regions having raisedrib-like elements.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the present invention, it is believed that thepresent invention will be better understood from the followingdescription in conjunction with the accompanying drawings, in which likereference numerals identify like elements and wherein:

FIG. 1 is a schematic illustration of an exemplary process for formingthe breathable film of the present invention;

FIG. 2 is an enlarged illustration of the opposing pairs of rolls of theprocess of FIG. 1;

FIG. 3 is a plan view illustration of a preferred embodiment of abreathable polymeric film having a first region and a second region ofthe present invention with the rib-like elements of the second regionfacing toward the viewer;

FIG. 3A is a segmented, perspective illustration of the breathable filmof FIG. 3 in an untensioned condition;

FIG. 4 is an enlarged photograph of the breathable film of the presentinvention, FIGS. 4A and 4B are enlarged photographs of the breathablefilms cross-section illustrating the formation of micropores in theportions of the film which have been stretched or thinned;

FIG. 5 is an illustration of an alternative embodiment of the opposingpairs of rolls of the process of FIG. 1;

FIG. 6 is an illustration of another embodiment of the opposing pairs ofrolls of the process of FIG. 1; and

FIG. 7 is a schematic illustration of an alternative process for formingthe breathable film of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

While the present invention will be described in the context of a methodfor producing a breathable film which is particularly well suited foruse as a backsheet on a disposable absorbent article such as adisposable diaper, sanitary napkin, or bandage the present invention isin no way limited to such application. The detailed description of themethod herein will allow one skilled in the art to readily adapt thepresent invention to other applications.

Referring to FIG. 1 there is schematically illustrated at 10 a processfor forming a breathable material of the present invention. According tothe present invention, a film 12 is unwound from supply roll 11 andtravels in the direction indicated by the arrows associated therewith asthe supply roll 11 rotates in the direction indicated by the arrowsassociated therewith. The film 12 comprises a blend of a thermoplasticpolymer with an inorganic material. Suitable thermoplastic polymersinclude polyolefins such as polyethylenes, including linear low densitypolyethylene (LLDPE), low density polyethylene (LDPE), ultra low densitypolyethylene (ULDPE), high density polyethylene (HDPE), or polypropyleneand blends thereof with the above and other materials. Examples of othersuitable thermoplastic polymers which may also be used include, but arenot limited to, polyester, polyurethanes, compostable or biodegradablepolymers, heat shrink polymers, thermoplastic elastomers, andmetallocene catalyst-based polymers (e.g., INSITE® available from DowChemical Company and Exxact® available from Exxon).

The inorganic material or filler is selected from the group consistingof calcium carbonate, clay and titanium dioxide, with the preferredinorganic filler being calcium carbonate. Inorganic fillers having aparticle size of from about 1 to about 5 microns and constitute of fromabout 5 to about 70 percent by weight of the blend, preferably fromabout 20 to about 60 percent by weight of the blend. The inorganicfiller may be coated with a fatty acid ester to obtain higher loadingsin the polymer.

The inorganic filler and the thermoplastic polymer are blended togetherto form a homogeneous mixture in a suitable mixing extruder, or in aseparate preliminary compounding step. The mixture is then cast or blowninto a film.

From the supply roll 11 the film 12 is fed through a nip 51 of a pair ofopposing rolls 52 and 54 of apparatus 50. The apparatus 50 is moreclearly shown in FIG. 2. Roll 52 comprises a plurality of toothedregions 56 and grooved regions 58 extending about the circumference ofthe roll. The toothed regions 56 include a plurality of teeth 57. Teeth57 are substantially parallel to the axis of roll 52. Roll 54 comprisesa plurality of toothed regions 60 and grooved regions 62 extending aboutthe circumference of the roll. The toothed regions 60 include aplurality of teeth 61. Teeth 61 are substantially parallel to the axisof roll 54. The grooved regions 58 of roll 52 are aligned with thegrooved regions 62 of roll 54. Similarly, the toothed regions 56 of roll52 are aligned with the toothed regions 60 of roll 54 permitting teeth57 of roll 52 to engage or intermesh with teeth 61 of roll 54. Theportions of the film 12 passing between the grooved regions 58 and 62will be unformed, i.e., the film 12 will not be formed or stretched andwill remain substantially planar, while the portions of the film passingbetween toothed regions 56 and 60 will be formed and/or stretchedproducing rib-like elements in the film.

Conventional drive means and other conventional devices which may beutilized in conjunction with the apparatus of FIG. 1 are well known and,for purposes of clarity, have not been illustrated in the schematic viewof Fig. 1.

The above sequence may be repeated any number of times until the desiredproperties are obtained. Furthermore, the degree of engagement ofopposing rolls, the dimension, shape and spacing of the teeth on theopposing rolls, and the size of the rolls can be chosen to produce thedesired properties.

Referring now to FIG. 3 there is illustrated a preferred embodiment of abreathable film 12 after having passed through the pair of opposingrolls 52 and 54 of apparatus 50. The breathable film 12 is shown in FIG.3 in its substantially untensioned condition. The breathable film 12 isparticularly well suited for use as a backsheet on an absorbent article,such as a sanitary napkin or a disposable diaper. The film 12 has twocenterlines, a longitudinal centerline, which is also referred tohereinafter as an axis, line, or direction “1” and a transverse orlateral centerline, which is also referred to hereinafter as an axis,line, or direction “t”. The transverse centerline “t” is generallyperpendicular to the longitudinal centerline “1”.

Referring now to FIGS. 3 and 3A, film 12 includes a network of distinctregions. The network includes at least a first region 64 and a secondregion 66. Film 12 includes a transitional region 65 which is at theinterface between the first region 64 and the second region 66. Film 12has a first surface 52 a and an opposing second surface 52 b. In theembodiment shown in FIGS. 3 and 3A, the film includes a plurality offirst regions 64 and a plurality of second regions 66. The first regions64 have a first axis 68 and a second axis 69, wherein the first axis 68is preferably longer than the second axis 69. The first axis 68 of thefirst region 64 is substantially parallel to the longitudinal axis ofthe film 12 while the second axis 69 is substantially parallel to thetransverse axis of the film 12. The second regions 66 have a first axis70 and a second axis 71. The first axis 70 is substantially parallel tothe longitudinal axis of the film 12, while the second axis 71 issubstantially parallel to the transverse axis of the film 12. In thepreferred embodiment of FIG. 3, the first regions 64 and the secondregions 66 are substantially linear, extending continuously in adirection substantially parallel to the longitudinal axis of the film12.

The film 12 is comprised of at least a first region and a second region,wherein the first region is visually distinct from the second region. Asused herein, the term “visually distinct” refers to features of the filmwhich are readily discernible to the normal naked eye when the film orobjects embodying the film are subjected to normal use.

In the preferred embodiment shown in FIGS. 3 and 3A, the first regions64 are substantially planar. That is, the material within the firstregion 64 is in substantially the same condition before and after theformation step undergone by film 12. The second regions 66 include aplurality of raised rib-like elements 74. The rib-like elements 74 havea first or major axis 76 which is substantially parallel to thetransverse axis of the film 12 and a second or minor axis 77 which issubstantially parallel to the longitudinal axis of the film 12. Thefirst axis 76 of the rib-like elements 74 is at least equal to, andpreferably longer than the second axis 77. Preferably, the ratio of thefirst axis 76 to the second axis 77 is at least about 1:1 or greater,and more preferably at least about 2:1 or greater.

The rib-like elements 74 in the second region 66 may be separated fromone another by unformed areas. Preferably, the rib-like elements 74 areadjacent one another and are separated by an unformed area of less than0.10 inches as measured perpendicular to the major axis 76 of therib-like elements 74, and more preferably, the rib-like elements 74 arecontiguous having no unformed areas between them. The dimension of therib-like elements can also be varied. A more detailed description of afilm having first and second regions as shown in FIGS. 3 and 3A can befound in U.S. Pat. No. 5,518,801 issued to Chappell et. al. on May, 21,1996which is incorporated herein by reference.

Referring now to FIG. 4 there is shown an enlarged photograph of thefilm 12 after having passed through a pair of opposing rolls of thepresent invention, like opposing rolls 52 and 54 of apparatus 50. As canbe seen in FIG. 4, the film 12 includes substantially planar firstregions 64 and second regions 66 which include raised rib-like elements74. The portions of the film 12 forming the rib-like elements 74 of thesecond region 66 have been stretched or thinned as opposed to thethicker first regions 64 which have not been stretched or thinned. Whilethe second regions 66 have been stretched or thinned, the stretching orthinning is not necessarily uniform throughout the second regions 66.

FIGS. 4A and 4B are enlarged photographs of portions of the films incross-section. The portion of the film in FIG. 4A has been stretched toa greater degree than the portion of the film in FIG. 4B and as a resulthas a thickness dimension less than that of the portion of the film inFIG. 4B. In addition to having a reduced thickness, the film shown inFIG. 4A has an increased pore volume compared to the film shown in FIG.4B. The stretching of the film separates the inorganic filler from thepolymer due to stress concentration creating pores in the film. Theformation of micropores permits the film to be breathable allowing thepassage of vapor therethrough while retarding the passage of liquid.

One measurement of breathability is the moisture vapor transmission rate(MVTR) of the material. In order to reduce humidity and he at buildupwithin the diaper, it has been found that the breathable film of thepresent invention when used as a backsheet on a disposable diaper shouldhave a moisture vapor transmission rate of from about 500 g/m²/24 hr. toabout 5000 g/m²/24 hr., more preferably from about 1000 g/m²/24 hr. toabout 5000 g/m²/24 hr., and most preferably from about 2000 g/m²/24 hr.to about 5000 g/m²/24 hr.

The moisture vapor transmission rate is measured by the method set forthbelow. A known amount of CaCl₂ is put into a flanged cup. A samplematerial is placed on top of the cup and held securely by a retainingring and gasket. The assembly is then weighed and recorded as theinitial weight. The assembly is placed in a constant temperature (40°C.) and humidity (75% RH) chamber for five (5) hours. The assembly isthen removed from the chamber and allowed to equilibrate for at least 30minutes at the temperature of the room where the balance is located. Theassembly is then weighed and recorded as the final weight. The moisturevapor transmission rate is calculated and expressed in g/m²/24 hr. usingthe following formula:${MVTR} = \frac{\left( {{{final}\quad {weight}} - {{initial}\quad {weight}}} \right) \times 24.0}{{area}\quad {of}\quad {sample}\quad {in}\quad {meters} \times 5.0\quad \left( {{time}\quad {in}\quad {chamber}} \right)}$

Another measurement of breathability is the oxygen permeation rate (OPR)of the material. In order to reduce humidity and heat buildup within thediaper, it has been found that the breathable film of the presentinvention when used as a backsheet on a disposable diaper should have aoxygen permeation rate of from about 3 cu. meters/sq. meter/24 hrs. toabout 20 cu. meters/sq. meter/24 hrs.

The oxygen permeation rate is measured by the method set forth below.The rate of oxygen permeation through a material is measured using aglass chamber with an oxygen monitor and probe. A sample is cutapproximately equal in diameter to an aluminum foil disc having a 12 cmdiameter, Mocon Part #025-493. The sample is placed on top of the disc.The glass chamber is placed on the disc and a circle is traced. Thecircle in the sample is cut. Once prepared, the sample disc (foil sidedown) is clamped between the glass chamber containing the probe and thecenter glass piece. A circular piece of rubber is placed between thecenter glass piece and the open ended glass chamber and sealed with asecond clamp. The chamber is flushed with nitrogen to a concentration of0.1% oxygen on both sides of the film. One side is then exposed to air(20.9% oxygen), and the times required to reach predetermined oxygenconcentrations on the opposite side of the film are recorded. Thepermeation rate of oxygen through the sample is then calculated usingthe following formula:${OPR} = \frac{\left( {5.57*10^{4}} \right)\left( {{volume}\quad ({mL})} \right)}{\left( {{average}\quad {time}\quad \left( \sec \right)} \right)}$

Where:

(5.57*10⁴)=Constant value incorporating the slope (time in minutes vs %oxygen level change (0.5%)), 1% oxygen level of interest, conversion ofseconds to minutes, conversion of minutes to days and surface area ofsample (0.775 in²);

Volume=Measured volume of the glass chamber containing the oxygen probe;

Average time=Average value of the time interval associated with a 0.5%increase in oxygen concentration.

Prior to undergoing the process of the present invention the blendedfilm has a moisture vapor transmission rate of less than about 100g/m²/24 hr. and an oxygen permeation rate of less than about 0.5 cu.meters/sq. meter/24 hrs. Thus blended film does not provide the desiredbreathability achieved by the film after having undergone the process ofthe present invention.

As seen in FIGS. 3A and 4, the method of the present invention expandsthe film out of the x-y plane, i.e., in the z direction to provide thefilm with an increased caliper via the raised rib-like elements. Incontrast, traditional incremental stretching equipment, such asdisclosed in U.S. Pat. No. 4,116,892 issued to Schwarz on Sep. 26, 1978,expands the film in the x-y plane, in either the longitudinal ortransverse directions or both. Thus, traditional incremental stretchingequipment produces a film of larger planar dimension, e.g., a filmhaving a larger width and/or length dimension. Producing a film oflarger dimension may be undesirable in certain circumstances as this mayrequire modifying existing production equipment to handle the largerfilm.

In addition to providing the film with breathability, the process of thepresent invention provides a film having a cloth-like appearance. Thisis particularly important if the film is to be used as a backsheet on adisposable diaper. Furthermore, the present invention may also providethe breathable film with elastic-like properties allowing the film tostretch under relatively low forces. The ability to stretch under normalwearing forces can be very beneficial for disposable diapers to provideincreased comfort and improved fit for the wearer.

As opposed to being unwound from a supply roll as shown in FIG. 1, thefilm 12 may be fed through the nip 51 of opposing rolls 52 and 54without first being stored on a supply roll. For example, the film 12may be fed through the nip 51 directly from an extruder.

After the film 12 has been fed through the nip 51 of opposing rolls 52and 54 the film 12 may be wound up onto a take-up roll. Alternatively,the film may be fed directly to a disposable absorbent article converterwhere it forms the backsheet of a disposable absorbent article.

Examples of disposable absorbent articles are described in U.S. Pat. No.4,950,264, issued to Osborn on Aug. 21, 1990; U.S. Pat. No. 4,425,130,issued to DesMarais on Jan. 10, 1984; U.S. Pat. No. 4,321,924, issued toAhr on Mar. 30, 1982; U.S. Pat. No. 4,589,876, issued to Van Tilburg onAug. 18, 1987; U.S. Pat. No. 3,860,003, issued to Kenneth B. Buell onJan. 14, 1975; and U.S. Pat. No. 5,151,092 issued to Kenneth B. Buell etal. on Sept. 29, 1992. Each of these patents are hereby incorporatedherein by reference.

While the entire portion of the film may be subjected to the formingoperation, the present invention may also be practiced by subjecting toformation only a portion thereof For example, it may be desirable toprovide specific portions of the diaper backsheet with breathability,such as the side portions while leaving other portions, such as thecentral portion of the backsheet nonbreathable.

After being formed as described above, the breathable film of thepresent invention may be secured to other materials to form a laminate.Preferably, the breathable film of the present invention is secured to anonwoven web. The nonwoven web may comprise natural fibers (e.g., cottonor wood fibers), or may comprise synthetic fibers such as polyethylene,polypropylene, polyester, or any combination of such fibers. Thenonwoven web may be carded, spunmelt, meltblown, or air-through bonded,or have any other characteristic or be manufactured in any manner knownin the art. Nonwoven webs are preferred when the laminate is to be usedas a diaper backsheet as the nonwoven web forms the outermost cover ofthe diaper providing the diaper with a cloth-like look and feel.Further, the outer nonwoven web cover may provide the diaper with alanding zone capable of engaging the hooks of a hook and loop typefastener. Other embodiments are contemplated wherein the breathable filmis secured to other materials such as woven webs, foams, scrims, films,loose fibers, or any other material or combination that will give thediaper a cloth-like look and feel.

The laminates may be made by any number of methods known to thoseskilled in the art. Such bonding methods include but are not limited toheat bonds, adhesive bonds (using any of a number of adhesives includingbut not limited to spray adhesives, hot melt adhesives, latex basedadhesives and the. like), pressure bonds, ultrasonic bonds, dynamicmechanical bonds, sonic bonds, or any other suitable attachmenttechnique or combinations of these attachment techniques as are known inthe art.

Alternatively, the blended film may first be secured to another materialto form a laminate prior to being formed according to the method of thepresent invention as described above. The laminate may comprise any ofthe aforementioned structures. In an especially preferred embodiment,the laminate is formed by extruding the breathable blend onto a nonwovenweb. The blended film is extruded directly onto a nonwoven web whilestill in a partially molten state, thereby bonding the blended film toone side of the nonwoven web. The laminate may then be subjected to theprocess of the present invention. Alternatively, the laminate may beformed by vacuum laminating a nonwoven web to the breathable blend. Thelaminate may then be subjected to the process of the present invention.

Referring now to FIG. 5 there is shown an illustration of an alternativeembodiment of the opposing pairs of rolls of the process of FIG. 1. Fromthe supply roll 11 the film 12 is fed through a nip 151 of a pair ofopposing rolls 152 and 154 of apparatus 150. Roll 152 comprises aplurality of toothed regions 156 and grooved regions 158 extending aboutthe circumference of the roll. The toothed regions 156 include aplurality of teeth 157. Teeth 157 are substantially parallel to the axisof roll 152. Roll 154 comprises a plurality of teeth 161 extending aboutthe circumference of the roll. Teeth 161 are substantially parallel tothe axis of roll 154. Teeth 157 of roll 152 engage or intermesh withteeth 161 of roll 154. The portions of the film 12 passing between thegrooved regions 58 of roll 152 and the teeth 161 of roll 154 will beunformed, i.e., the film 12 will not be formed or stretched and willremain substantially planar, while the portions of the film passingbetween toothed regions 156 comprising teeth 157 and the teeth 161 ofroll 154 will be formed and/or stretched producing rib-like elements inthe film.

Referring now to FIG. 6 there is shown an illustration of an alternativeembodiment of the opposing pairs of rolls of the process of FIG. 1. Fromthe supply roll 11 the film 12 is fed through a nip 251 of a pair ofopposing rolls 252 and 254 of apparatus 250. Roll 252 comprises aplurality of toothed regions 256 and grooved regions 258 that extendparallel to the axis of roll 252. The toothed regions 256 include aplurality of teeth 257. Teeth 257 are substantially perpendicular to theaxis of roll 252. Roll 254 comprises a plurality of teeth 261 extendingabout the circumference of the roll. Teeth 261 are substantiallyperpendicular to the axis of roll 254. Teeth 257 of roll 252 engage orintermesh with teeth 261 of roll 254. The portions of the film 12passing between the grooved regions 258 of roll 252 and the teeth 261 ofroll 254 will be unformed, i.e., the film 12 will not be formed orstretched and will remain substantially planar, while the portions ofthe film passing between toothed regions 256 comprising teeth 257 andthe teeth 261 of roll 254 will be formed and/or stretched producingrib-like elements in the film. FIG. 7 is a schematic illustration of analternative process for forming the breathable film of the presentinvention. According to the present invention, a film 312 is unwoundfrom supply roll 311 and travels in the direction indicated by thearrows associated therewith as the supply roll 311 rotates in thedirection indicated by the arrows associated therewith. The film 312comprises a blend of a thermoplastic polymer with an inorganic material.From the supply roll 311 the film 312 is fed through a nip 351 of a pairof opposing rolls 352 and 354 of apparatus 350. The opposing rolls 352and 354 may be any one of those shown in any one of FIGS. 2, 5 or 6.After having been formed by apparatus 350 the film 312 is fed through anip of a pair of opposing rolls 452 and 454 of apparatus 450. Theopposing rolls 452 and 454 may be any one of those shown in any one ofFIGS. 2, 5 or 6.

The above sequence may be repeated any number of times until the desiredproperties are obtained. Furthermore, the degree of engagement ofopposing rolls, the dimension, shape and spacing of the teeth on theopposing rolls, and the size of the rolls can be chosen to produce thedesired properties.

In an alternative embodiment, the process of the present invention maybe practiced in combination with the incremental stretching operationdisclosed in the aforementioned U.S. Pat. No. 4,116,892 issued toSchwarz on Sep. 26, 1978, which is incorporated herein by reference. Forexample, the film may be incrementally stretched as disclosed in Schwarzand then fed through the opposing rolls of the present invention. Theabove sequence may be reversed and/or repeated to produce the desiredproperties in the film.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

What is claimed:
 1. A backsheet for a disposable absorbent articlewherein micropores are formed in said backsheet to render said backsheetbreathable.